1. Field of the Invention
The present invention relates to a method of processing the surface of a semiconductor substrate with plasma for manufacturing a semiconductor device and a plasma processing apparatus.
2. Description of the Background Art
Electronics has remarkably developed in recent years, considerably owing to development of semiconductor devices, i.e., progress of semiconductor equipment and semiconductor technology in the final analysis.
In a series of steps for completing a semiconductor device from a semiconductor substrate such as a silicon wafer, i.e., in the so-called wafer process, various processing is sequentially performed so that a semiconductor wafer (hereinafter simply referred to as a wafer) is processed in various semiconductor equipment and completed as a product.
As the degree of integration of a semiconductor device is increased to require processing of a fine shape, the necessity for a method of and an apparatus for correctly controlling various wafer processing steps (hereinafter simply referred to as a process) respectively are increased.
Particularly in etching employing plasma, process control comprehending its reaction mechanism is now becoming indispensable, and it is an important subject to process a fine pattern in high anisotropy while maintaining uniformity of an etching rate in a wafer plane, an etching shape and the like following diameter enlargement of the wafer.
With reference to FIG. 47, the structure of an etching apparatus disclosed in Japanese Patent Publication No. 4-73287 (1992) is now described as a conventional etching apparatus employing plasma.
In the etching apparatus shown in FIG. 47, an upper electrode 4 and a lower electrode 2 are arranged on an upper portion and a lower portion of a processing chamber 1 formed by a vacuum vessel so that main surfaces thereof are opposed to each other, while the upper electrode 4 is grounded and the lower electrode 2 is connected with a high-frequency (e.g., 13.56 MHz) power source 9 through a blocking capacitor 8.
The lower electrode 2 also serves as an object mounting base mounting an object (wafer) 5 thereon, and is electrically insulated from the processing chamber 1 by an insulator 3.
A gas inlet port 6 of the processing chamber 1 is connected with a gas supply system GL supplying wafer processing gases (may be liquids) for dry etching. The gas supply system GL includes gas sources GS1 and GS2 and needle valves V1 and V2 for controlling the gas flow rates mounted on the respective gas sources GS1 and GS2. A controller VC controls the needle valves V1 and V2, for adjusting the gas supply quantities.
The processing chamber 1 is connected with an evacuation port 7 evacuating the processing chamber 1 to a prescribed pressure level and a pressure measurer (not shown) measuring the pressure of a discharge space in the processing chamber 1.
An etching operation is now described. A wafer 5 introduced into the processing chamber 1 by a transfer unit (not shown) is placed on the lower electrode 2. Then, the wafer processing gases are introduced into the processing chamber 1 through the gas inlet port 6 while controlling the needle valves V1 and V2 with the controller VC to attain prescribed gas flow rates.
In general, the pressure measurer and a discharge conductance controller (not shown) regulate the pressure in the processing chamber 1 to a prescribed level while introducing the wafer processing gases.
After the pressure regulation, high-frequency power is applied to the lower electrode 2 from the high-frequency power source 9, to cause RF glow discharge between the lower electrode 2 and the upper electrode 4. The wafer processing gases in the processing chamber 1 are converted to plasma due to the RF glow discharge, and active gas particles (hereinafter referred to as radicals) and ions generated in the plasma react with the wafer 5 to progress etching. Volatile etching reaction products resulting from the reaction with the wafer 5 are discharged from the evacuation port 7.
If the ions and radicals are ununiformly supplied to the wafer 5, therefore, the etching rate in the wafer plane is not uniform. Thus, uniformity of the plasma generating ions and radicals (hereinafter referred to as ions.radicals) is important. If the etching reaction products are ununiformly discharged on the wafer surface, further, the ions.radicals are ununiformly supplied to the wafer 5, consequently the etching rate in the wafer plane is non uniform.
The etching process is now described with reference to an aluminum alloy (e.g., Al--Si, Al--Cu, Al--Si--Cu or the like) most generally employed as a metal wire material in a semiconductor device at present.
In general, such an aluminum alloy is subjected to dry etching with a gas (chloric gas) mainly composed of chlorine. Aluminum readily spontaneously reacts with chlorine, and etching progresses chemically and anisotropically. This reaction requires no ions.radicals in plasma.
In order to obtain an anisotropic shape required for a fine pattern, therefore, a method of forming a protective film (hereinafter referred to as a side wall protective film) on a side wall of a wire during etching reaction thereby preventing isotropic (transverse) etching with the chloric gas is employed in general.
The side wall protective film may be formed through a decomposition product from photoresist forming an etching mask. The decomposition product from the photoresist results from physical sputtering of the photoresist with ions in plasma or chemical reaction between radicals (active species) in the plasma and the photoresist. Thus, it follows that the side wall protective film is automatically formed during etching.
The side wall protective film may alternatively be formed through a deposits formed during etching. In this case, a sedimentary gas suitable for forming a protective film is added to wafer processing gases thereby prompting formation of a deposits and automatically forming a side wall protective film with the deposits.
The former method has such advantages that the number of types of used gases (generally combination of Cl.sub.2 /BCl.sub.3) may be small and the amount of the reaction products adhering to the inner wall of the processing chamber 1 is relatively small. However, the side wall protective film is formed while consuming the photoresist forming the mask, and hence this method has such disadvantages that etching must be performed under conditions remarkably consuming the photoresist, the etching selection ratio between the photoresist and the aluminum alloy is reduced and the performance is deteriorated in view of refinement.
In the latter method, on the other hand, the reaction products from the sedimentary gas facilitate formation of the side wall protective film, which is formed also when etching is performed under conditions consuming only a small amount of photoresist. Thus, isotropic etching with the chloric gas is prevented to enable anisotropic etching, while the etching selection ratio of the photoresist and the aluminum alloy can be improved. Along with the chloric gas, however, a sedimentary gas such as N.sub.2, CHF.sub.3 or CH.sub.2 F.sub.2 must be employed singularly or in combination. Thus, this method has disadvantages such as reduction of the etching rate, occurrence of foreign matter resulting from adhesion of the reaction products to the inner wall of the processing chamber, occurrence of a side wall residue after anticorrosion treatment.
In the etching process with the conventional etching apparatus, as hereinabove described, isotropic etching with the chloric gas is prevented by forming the side wall protective film through the decomposition product from the photoresist or the deposits formed during etching. As the diameter of the wafer 5 is increased, however, the distance between the central portion of the wafer 5 and the evacuation port 7 is so increased that the evacuate efficiency for the reaction products resulting from etching reaction is remarkably lowered at the wafer central portion as compared with the peripheral portion of the wafer, the reaction products reside and ions.radicals or the like hardly reach the wafer surface.
On the peripheral portion of the wafer 5, on the other hand, the reaction products are efficiently evacuated regardless of the diameter of the wafer 5, and ions.radicals or the like readily reach the wafer surface.
As the diameter of the wafer 5 is increased, therefore, the component ratio, the absolute amount etc. of ions.radicals or the like reaching the wafer surface remarkably vary with the central portion and the peripheral portion of the wafer 5 to often result in such a phenomenon that the etching rate, the etching shape and the like remarkably vary with the central portion and the peripheral portion of the wafer 5, leading to a problem in manufacturing of a semiconductor device with a wafer having a large diameter. This is the first problem.
Further, refinement of the etching pattern progresses following increase of the diameter of the wafer, to cause a new problem. When the aspect ratio (a/b: a stands for the etching depth and b stands for the etching width) is increased, the reaction products are hardly separated (evacuated) on the bottom portion of the pattern to inhibit etching reaction, to result in a phenomenon reducing the etching rate (a phenomenon called microloading or RIE lag, hereinafter simply referred to as RIE lag for the purpose of simplification). This is the second problem.
The first and second problems remarkably depend on the uniformity of plasma, and efforts are being made to uniformly the plasma concentration (in other words, ion.radical concentration) in the vicinity of the wafer. When etching an aluminum alloy with chloric gas, however, aluminum readily spontaneously reacts with chlorine as hereinabove described, and the etching reaction progresses with no requirement for ions.radicals in plasma. Thus, it is difficult to solve the aforementioned problems due to remarkable dependency on the gas concentration in the vicinity of the wafer surface.